The invention relates to a star coupler for a bus system and to a bus system in which such a star coupler is used. It also relates to a method for interchanging signals in a bus system, in which a plurality of bus branches are coupled via a star coupler.
A star coupler is the central element of a star connection. A plurality of bus branches are connected to such a star coupler. Each bus branch comprises at least one control device, in which case at least some of the control devices can transmit and some of the control devices can receive, and preferably at least some of the control devices can both transmit and receive. The task of a star coupler is to forward the signals arriving from one branch to the other branches. In this case, an active star coupler amplifies the signals. For example, DE 10 2005 061 395 A1 discloses an active star point for use in a communication system with star topology, which star point amplifies incoming analog signals with the aid of an operational amplifier and forwards said signals. Said document mentions the fact that an incoming data signal can be distributed to one or more of the remaining branches. However, on account of the purely analog method of operation of the active star point from DE 10 2005 061 395 A1, said star point is not suitable for use in time-controlled systems.
In time-controlled bus systems, for instance in FlexRay systems or TTP/C systems used in motor vehicles, a cycle time is divided into a plurality of time slots. Only a particular control device is allowed to transmit in each time slot so that there is no signal interference with the signals from another control device. Such time-controlled systems contain so-called bus guardians which check whether a user (control device) of the bus system is even allowed to transmit at the respective time, that is to say in a current time slot. The bus guardian also determines whether the message is allowed to be distributed. Such a bus guardian can be integrated in a star coupler. In time-controlled systems, such a star coupler usually includes cascaded transceivers for each branch. From the point of view of the star coupler, such a transceiver is respectively assigned to a plurality of connections to which bus branches can be connected. Transceivers convert analog signals arriving via the respective associated connection into digital signals and convert signals running to the respective associated connection from digital signals into analog signals. The bus guardian function can be used in the phase in which the signals have been converted into digital signals and have not yet been converted back into analog signals. The star coupler then typically comprises a timer in order to be able to distinguish individual time slots from one another. With an increasing number of bus users, buses reach their limit in terms of their transmission capacity. For example, a FlexRay bus has a bus bandwidth of 10 Mbits. It is desirable to use this bandwidth as optimally as possible. For this purpose, there has been a move toward dividing the bus system into individual clusters, the bus users in a cluster being allowed to transmit in time slots defined for the cluster. Data can be interchanged between different clusters only if a so-called gateway is used. In time-controlled systems, at least two synchronization nodes must be present in each cluster system. A separate bus guardian must be used in each cluster in order to safeguard communication.